This invention relates to a solid state device and method for measuring the concentration of oxygen in a substance to which the device is exposed.
The need for measuring the concentration of oxygen in biological fluids has led to the development of several oxygen sensing devices including the so-called Clark oxygen electrode. This device includes an electrode assembly in which is contained a metal cathode, an electrolyte solution surrounding the metal cathode, a reference anode also disposed in the electrolyte, and an oxygen-permeable membrane stretched across an opening in the electrode assembly to contain the electrolyte and yet to allow diffusion therethrough of oxygen to which the electrode assembly is exposed. Such oxygen also then diffuses through the electrolyte to the cathode where, as a result of reduction of the oxygen, a current is produced, with this current being proportional to the extent of oxygen reduction. This type of device relies upon the mass transport of oxygen to the cathode and because of this, the device is sensitive to any factor which would affect mass transport such as temperature, amount of electrolyte through which the oxygen must diffuse, external pressure, accumulation of products of the electrochemical reactions, etc. In other words, the Clark electrode is a nonequilibrium device so that with the passage of time, the readout of the device may change and possibly introduce error. Also, the response time is fairly long because of the distance over which the oxygen molecules must diffuse and thus the time needed for the oxygen to diffuse.
There are a number of other oxygen sensing devices which can only operate at high temperatures--generally above 200.degree. C.--because of the materials used in the devices. These devices (typically referred to as "high temperature oxygen sensors"), of course, are limited to high temperature applications.
In U.S. Pat. No. 4,020,830 issued May 3, 1977, a chemical sensitive field-effect transistor transducer is disclosed wherein such transducer has the capability of selectively detecting and measuring chemical properties of substances. This type of transducer eliminates the need for an electrolyte which, of course, simplifies fabrication of the device. This device utilizes a field-effect transistor arrangement for simply and efficiently measuring various chemical properties of substances. Specific embodiments for sensing oxygen were not discussed in the patent, however.
In view of this prior art, it would appear desirable to provide a solid state structure for measuring the concentration of oxygen in a substance, where such structure would not be influenced by mass transport problems or temperature limitations.